The present invention relates to a process for the manufacture of a gas-permeable hard contact lens and a gas-permeable hard contact lens. More specifically, it relates to a process for efficiently producing a gas-permeable hard contact lens having excellent durability strength and a gas-permeable hard contact lens which is improved in durability strength and free of optical strains and which has no problem on wearing.
A contact lens is generally classified into a hard contact lens and a soft contact lens. Further, the hard contact lens is classified into an oxygen-non-permeable hard contact lens formed of a homopolymer or copolymer of methyl methacrylate (MMA) and a rigid gas permeable (RGP) hard contact lens formed of a copolymer formed of siloxanyl methacrylate (SiMA), MMA and fluoroalkylmethacrylate (FMA) as main components.
In the beginning, the main stream of a hard contact lens was an oxygen-non-permeable hard contact lens formed of polymethyl methacrylate (PMMA) having excellent biocompatibility and excellent transparency. Concerning the above oxygen-non-permeable hard contact lens, however, as contact lenses have come to be used widely, damage is caused on corneal epithelium since the wearing time period thereof is extended, or influences which the long wearing time period thereof has on cells of corneal endothelium have come to be discussed. Hard contact lenses with more safety have come to be developed.
The gas-permeable hard contact lens is a hard contact lens developed under the above circumstances, and it is classified into a low-oxygen-permeable type and a high-oxygen-permeable type depending upon an oxygen permeability coefficient (DK value). In present markets, the main stream is a high-oxygen-permeable type (continuous-wearing contact lens) due to a further extended wearing time period.
Since, however, the gas-permeable hard contact lens has molecular-level holes for allowing oxygen gas necessary for cornea wearing the contact lens to permeate therethrough and exhausting metabolically generated carbon dioxide gas, it has a defect that its durability strength against an instantanious impact or bending decreases inevitably with an increase in the DK value.
For overcoming the above defect, there is proposed an oxygen-permeable hard contact lens which has higher compression fracture strength than a convention contact lens and is not easily broken (JP-A4-67117). In the above method, however, it is required to carry out a polymerization so as to attain a uniform polymerization not only by controlling the polymerization rate of monomers used but also by precisely controlling polymerization conditions. It is therefore very difficult to control the polymerization conditions.
There is also proposed a methods in which a non-crosslinked polymer formed by polymerizing monomers containing at least one monomer selected from an unsaturated carboxylic acid, an unsaturated carboxylate ester or an unsaturated carboxylic acid anhydride is compression-molded under heat in the presence of a polyamine, to produce a transparent optical resin molded article improved in solvent resistance and mechanical strength (JP-A-7-62022). In the above method, the molded article obtained in excellent solvent resistance, free of optical strains, homogeneous and excellent in transparency since a non-crosslinked powdery polymer is crosslinked in the presence of polyaminde during the compression molding under heat, and further, it undergoes almost no change of a form with the passage of time since the internal stress generated in the polymer after the molding is very small. However, due to a diversity of base curves of hard contact lenses and due to a variety of diopters and a variety of diameters, the above method has a defect that the number of molds therefor increases and that it requires immense labor to manage them.
Further, there is proposed a method in which a contact lens material is compression-molded under heat to produce a contact optical molded article (JP-A-60-49906 and JP-A-61-41118). In this method, a film having a weight of a molded article to be produced and having a uniform thickness is punched out or cut, and the resultant piece is place between dies having convex surfaces correspond to the form of a molded article or dies having convex and concave surfaces and a re-molded under pressure at a temperature higher than the glass transition temperature of a thermoplastic material used but lower than the melt-flowing temperature thereof, to produce a contact optical molded article. In the above method, however, a film-shaped non-processed produce which is formed of a thermoplastic resin having no crosslinked structure and has a weight equivalent to the weight of a molded article is molded to a finished form, and the molded article obtained has a problem that the form thereof changes with the passage of time or that the strength thereof is not sufficient since it has no crosslinked structure.
Supplying oxygen to cornea wearing a contact lens naturally alleviates a physiological burden on the cornea, and it is said that the deficiently of oxygen in cornea has a clinically large influence. A contact lens is required to secure sufficient safety for a long period of time since it is to be in direct contact with a living tissue, cornea, which is highly sensitive and essential for the function of vision. Further, since cornea constantly requires oxygen for maintaining transparency, a composition of monomers to be contained, or the like, is devised for increasing the DK value. However, when the DK value is increased, there is caused a problem that the durability decreases. Further, copolymers which are improved in durability strength by adding a strength-imparting monomer or a crosslinked agent are available, while they cannot be said to be satisfactory.
Under the circumstances, it is a first object of the present invention to provide a process for efficiently producing a gas-permeable hard contact lens which is improved in durability strength, is free of optical strains and has no problem on wearing.
It is a second object of the present invention to provide a gas-permeable hard contact lens which is improved in durability strength, is free of optical strains and has no problem on wearing.
For achieving the above objects, the present inventors have made diligent studies and as a result have found that the above first object can be achieved by hot press-stretching a crosslinked gas-permeable hard contact lens material and then machining it.
Further, it has been found that the second object can be achieved by a gas-permeable hard contact lens obtained by the above process and a gas-permeable hard contact lens formed of a crosslinked gas-permeable hard contact lens material which is hot press-stretched so as to have a compression ratio and a compression-flexure fracture strength in specific ranges.
The present invention has been made on the basis of the above findings.
That is, the present invention provides:
(1) a process for the production of a gas-permeable hard contact lens, which comprises hot press-stretching a crosslinked gas-permeable hard contact lens material and then machining the press-stretched material,
(2) a gas-permeable hard contact lens obtained by the above process (1), and
(3) a gas-permeable hard contact lens formed of a crosslinked gas-permeable hard contact lens material which is not press-stretched to have a compression ratio of 5 to 50% and a compression-flexure fracture strength of 300 to 1,500 g.